Grid-stiffened structures are used in a wide variety of applications and configurations. These structures typically consist of a thin skin, or face-sheet, integrally connected to a series of rib or blade stiffeners. The face-sheet can be placed on either the internal or external portion of the structure. The rib-stiffeners typically form repeating patterns that create either rectangular or triangular cavities in the structure. Traditionally, these types of structures were machined from thick sheets of metallic material using large multi-head milling machines. Thus, a large portion of the metallic material was wasted as scrap.
Fiber-reinforced polymer composite materials can be used to make grid-stiffened structures that are more efficient than comparable metallic grid structures because the ribs can be made of unidirectional laminates. Hence, all of the fibers are oriented to run axially along the length of the ribs. The direction in which the highest stresses typically occur thus corresponds to the direction of highest strength and stiffness of the material.
The foregoing are known as advanced grid-stiffened structures, and their use has gained in popularity during the last decade because they eliminate or reduce the primary problems associated with honeycomb sandwich structures. More particularly, moisture uptake is a well-known problem for honeycomb panels because water tends to become trapped in the hexagonal cells of the honeycomb and causes corrosion and softening of the composite face-sheets. In contrast, grid-stiffened panels do not trap water since the panel has skin on one side only. A second critical problem with structures made of honeycomb sandwich panels is that a large amount of time is required to cut and splice the honeycomb core to fit on shapes having complex curvatures, such as aircraft fuselages and rocket payload fairings. This results in a high cost and a long lead-time for these structures. However, grid-stiffened panels can be manufactured using an almost entirely automated process. This may result in a cost savings of nearly 20% over a comparable honeycomb sandwich shroud.
A number of previous patents have been awarded for the fabrication of grid-stiffened composite structures. U.S. Pat. No. 6,007,894 teaches a technique for fabricating a grid-stiffened composite structure using a hand lay-up process.
U.S. Pat. No. 6,245,274 discloses a method for manufacturing advanced grid-stiffened structures that utilizes hybrid tooling. This method uses a rigid base tool to provide for the geometric shape and dimensional tolerance of the structure. Softer expansion tooling blocks having a high coefficient of thermal expansion are placed into grooves in the base tool. The tooling blocks have smaller grooves that fit around the uncured ribs and provide consolidation pressure on the structure during oven or autoclave curing. Either a filament winding machine or a fiber placement machine can be used to place the uncured tows of fiber into the grooves in the soft expansion tooling blocks.
U.S. Pat. No. 6,290,799 teaches a method for fabricating grid-stiffened structures utilizing a fiber placement machine. The foregoing method utilizes a rigid base tool with soft triangular-shaped expansion blocks where the expansion blocks are fastened to the base tool by pins at each of the corners. This reference discusses the possibility of placing the skin on both the inside and outside of the stiffening ribs. When the skin is found on the inside of the ribs, the skin is placed first on the bare base tool, then the uncured ribs are placed, and finally the expansion blocks are connected to a caul sheet that is then placed around the part for curing.
Each of these previously disclosed methods of fabricating grid-stiffened structures has inherent problems that make it too costly and complex to be competitive with traditional sandwich panel structures. For example, the hybrid expansion approach disclosed in U.S. Pat. No. 6,245,274 relies on a very complex base tool and matching set of expansion blocks. Once the base tool is fabricated, it is very difficult using this method to alter the location of the stiffening ribs to accommodate things such as windows, doors, and access panels. The complexity of the base tool makes it expensive and time consuming to fabricate, and the process of placing the individual expansion blocks into the grooves in the base tool is labor intensive. These factors detract from the suitability of using this method to fabricate very large structures such as aircraft fuselages and rocket payload fairings.
The hand lay-up fabrication method disclosed by U.S. Pat. No. 6,007,894 has the disadvantage of being useable only to make fairly small structures. Further, this method requires the use of skilled labor, which obviously increases the fabrication cost.
The process disclosed in U.S. Pat. No. 6,290,799 requires the expansion tooling blocks to be rigidly connected to either the base tool by pins or to the top caul sheet by adhesive or a co-cured bond. The problem with this approach is that the when the expansion tooling is rigidly constrained, a differential pressure along the length of the ribs is created during the curing process. This differential pressure can create variations in the thickness of the ribs as well as variations in porosity along the length of the ribs. These factors reduce the efficiency of the structure and render this manufacturing method undesirable.
There is a need in the art for a method of fabricating grid-stiffened structures that is lower in cost and less complex than the methodology of the prior art, as well as capable of producing structures with supporting ribs having constant thickness and porosity. The present invention fulfills this need in the art.